Photodissociation of CO₂^- in water clusters via Renner-Teller and conical interactions

The photochemistry of mass selected CO₂^-(H ₂O)_m, m=2-40 cluster anions is investigated using 266 nm photofragment spectroscopy and theoretical calculations. Similar to the previous 355 nm experiment [Habteyes et al., Chem. Phys. Lett. 424, 268 (2006)], the fragmentation at 266 nm yields two types of anionic products: O ^-(H₂O)_m-k (core-dissociation products) and CO₂^-(H₂O)_m-k (solvent-evaporation products). Despite the same product types, different electronic transitions and dissociation mechanisms are implicated at 355 and 266 nm. The 355 nm dissociation is initiated by excitation to the first excited electronic state of the CO₂^- cluster core, the 1 ²B ₁(²A″) state, and proceeds via a glancing Renner-Teller intersection with the ground electronic state at a linear geometry. The 266 nm dissociation involves the second excited electronic state of CO₂^-, the 2 ²A₁( ²A′) state, which exhibits a conical intersection with the 3 ²B₂(A′) state at a bent geometry. The asymptotic O^- based products are believed to be formed via this 3 ²B₂(A′) state. By analyzing the fragmentation results, the bond dissociation energy of CO₂^- to O ^-+CO in hydrated clusters (m ≥ 20) is estimated as 2.49 eV, compared to 3.46 eV for bare CO₂^-. The enthalpy of evaporation of one water molecule from asymptotically large CO₂ ^-(H₂O)_m clusters is determined to be 0.466±0.001 eV (45.0±0.1 kJ/mol). This result compares very favorably with the heat of evaporation of bulk water, 0.456 eV (43.98 kJ/mol).